Seismic method of performing the time picking step

ABSTRACT

The present invention provides a method of performing the time picking step in a VSP (vertical seismic profile) survey. In a preferred embodiment of the invention the time picking step is carried out on a combined three-component amplitude of the received seismic energy, which contains the amplitude of all the seismic energy received at the receiver. The amplitude of the direct pulse in the combined trace will not decrease to zero as the offset of the source is changed, as can be the case for the single-component amplitude of the direct pulse. In a particularly preferred embodiment of the invention, the combined three-component amplitude is calculated by summing the Hilbert instantaneous amplitudes of the x-, y- and components of the seismic data using the equation (I). The present invention also provides two new time picks. One time pick involves finding the maximum positive gradient of A(t). The other time pick entails extrapolating to A(t)=0 from the time at which A(t) has its greatest positive gradient, using the maximum positive gradient for the extrapolation.

The present invention relates to a method of processing seismic data, inparticular to the processing of seismic data acquired using a verticalseismic profile (VSP) seismic surveying method.

Seismic data are collected using an array of seismic sources and seismicreceivers. The data may be collected on land using, for example,explosive charges as sources and geophones as receivers, or the data maybe collected at sea using, for example, airguns as the sources andhydrophones as the receivers.

FIG. 1 is a schematic illustration of the survey geometry for the methodof seismic surveying known as vertical seismic profiling (VSP)surveying. In this surveying geometry, the receiver 1 is not disposed onthe earth's surface, but is disposed within the earth, in this examplewithin a borehole 6. The seismic source 2 is disposed on the earth'ssurface. Two ray paths for seismic energy are shown in FIG. 1. Path 3 isa path in which the seismic energy does not undergo reflection, althoughit is refracted at the boundary between two layers 7,8 of the earth.Since seismic energy that travels along this path travels direct fromthe source 2 to the receiver 1 without reflection, this path is known asthe “direct path”. Path 4 is a path in which seismic energy emitted bythe source 2 is incident on the receiver 1 after reflection by areflector 5 located at a greater depth than the receiver, and is thusknown as a “reflection path”.

In FIG. 1 the seismic source 2 is located at a distance from the pointat which the vertical line on which the receiver 1 is disposed passesthrough the earth's surface. This geometry is known as offset VSP, sincethere is a non-zero horizontal distance between the seismic source andthe receiver. The horizontal distance between the seismic source and thereceiver is generally known as “offset”. In an alternative VSP geometry,the seismic source is located vertically over the receiver, and this isknown as zero-offset VSP.

FIG. 1 shows only one seismic source and one receiver, but it ispossible for there to be more than one source and/or more than onereceiver. In the survey geometry known as multi-offset VSP, a pluralityof seismic sources are located on the surface of the earth, with eachsource having a different offset (i.e., being at a different horizontaldistance from the point at which the vertical line on which the receiver1 is disposed passes through the earth's surface).

One application of VSP seismic surveying is in “look-ahead” surveying.This form of seismic surveying is used during the drilling of aborehole. One or more seismic receivers are placed in the borehole,above the drilling head, and are used to gather information about thegeological structure beneath the drilling head. Decisions concerning thedrilling operation, for example determining the safe distance to drillbefore setting the next string of casing, are made on the basis ofinformation gathered about the underlying geological structure.

FIG. 2 is a schematic illustration of a seismic trace recorded by thereceiver in a VSP survey geometry. In FIG. 2 it is assumed that thesource emits a short pulse of seismic energy at time t=0. It will beseen that the amplitude of the seismic energy received at the receivervaries over time, and consists essentially of a number of pulsesseparated by periods of zero amplitude. The first pulse 9 in the tracecorresponds to the direct path of seismic energy from the source to thereceiver, since this path will have the lowest travel time of allpossible paths of seismic energy from the source to the receiver. Thesubsequent pulses correspond to energy paths that involve reflectionfrom reflectors at increasing depths within the earth, or to paths thatinvolve two or more reflections. The structure in the trace before thearrival of the direct pulse 10 is noise.

When a trace such as that shown in FIG. 2 is analysed, one importantstep in the analysis is the determination of the arrival time of thefirst pulse of seismic energy, which is the pulse transmitted over thedirect ray path. This step is generally known as the “time pickingstep”.

Although FIG. 2 shows just one seismic trace, in practice measurementswill be taken for a large number of different offsets between the sourceand the receiver. This will produce a series of a large number oftraces. As an example, FIG. 3(a) shows 80 traces with each tracecorresponding to a different offset. The large number of traces involvedin a VSP survey makes the time picking step one of the most timeconsuming and costly steps in the processing of VSP data.

Time picking algorithms are conventionally used to automate the timepicking step in the processing of VSP data. Conventional time pickingalgorithms operate on a single component of the seismic data, that is onthe amplitude of the seismic energy propagating in a single direction.

A seismic receiver is generally directional to some extent, and has afixed acceptance cone for seismic energy. However, as the offset isincreased the angle of incidence of the incoming direct pulse relativeto the receiver will change. The position of the seismic source relativeto the receiver may be chosen arbitrarily, and the azimuthal anglebetween the source and the receiver in principle can vary between 0 and90°. Moreover, the general VSP survey geometry may involve a wellboretrajectory that is not simply vertical but contains components in the x-or y-directions, and there is frequently no knowledge of the exactorientation of the receiver 1. It is thus possible for one particularcomponent of the direct pulse to have a measured amplitude of zero forsome positions of the seismic source relative to the receiver, if thedirection of the direct pulse falls outside the acceptance cone of thereceiver. This will cause a single component picking algorithm to fail,since the algorithm will wrongly identify another event in the seismictrace as the direct pulse.

One type of seismic receiver often used in a VSP survey is a 3Cgeophone, or 3 component geophone. A 3C geophone can record theamplitude of seismic energy propagating along three orthogonaldirections. In some cases the relationship between the three axes of thegeophone and the x-, y- and z-directions will be known, but this is notalways the case. Even if a three component geophone is used as theseismic receiver, it is possible that the geophone to be oriented insuch a way that the amplitude of the direct pulse as measured along oneaxis of the geophone becomes zero for some value of the offset. If thetime picking algorithm should operate on this component of the seismicenergy it will fail when the amplitude of the direct pulse in thiscomponent becomes zero.

This problem is illustrated in FIG. 3(a), which shows traces generatedby a receiver in a typical VSP survey. The traces show the amplitude inthe z-direction of seismic energy received at the receiver. The x-axisof FIG. 3(a) represents time and the y-axis of FIG. 3(a) represents theoffset between the source and the receiver. The seismic source isactuated at time t=0.

FIG. 3(a) shows traces obtained for 80 different offsets. It can be seenthat the time taken for the direct pulse to reach the receiver increasesas the offset between the receiver and the source increases. This isexpected, since the length of the direct path between the source and thereceiver will increase as the offset increases. However, it will be seenthat the amplitude of the direct arrival pulse is also affected by theincrease in offset. The amplitude of the z-axis component of the directpulse is seen to decrease and even change polarity as the offsetchanges. This is shown in more detail in FIG. 4(a), which is a partialenlarged view of FIG. 3(a).

The traces of FIG. 3(a) illustrate a situation in which a conventionalsingle component time picking algorithm is unsatisfactory. An algorithmthat attempted to determine the arrival time of the direct pulse fromthe traces shown in FIG. 3(a) would breakdown in the region where theamplitude of the z-component of the direct pulse falls to zero andreverses in polarity. Even if the time picking were carried out by eye,it would still be difficult to carry out accurately.

The present invention a method of processing seismic data comprising thesteps of: recording the amplitudes in at least first and seconddirections of seismic energy received at a seismic receiver as afunction of time, the first and second directions not being co-linear;generating a time-dependent combined amplitude A(t) of the seismic datafrom the amplitudes in the first and second directions; and determiningthe arrival time of a pulse of seismic energy from the combinedamplitude.

Even if the source and receiver should be oriented such that onecomponent of the amplitude of the direct pulse at the receiver has zeroamplitude, it is possible to generate a combined amplitude that willalways produce a positive amplitude for the direct arrival pulse. Bygenerating such a combined amplitude, and performing the time pickingstep on the combined amplitude, the problems involved with using asingle component algorithm are eliminated. The combined amplitude alwaysprovide a positive amplitude for the direct pulse, so that an algorithmthat looks for the direct arrival pulse in the combined amplitude willnot be affected if one of the components of the amplitude should bezero.

In a preferred embodiment, the method further comprises recording theamplitude in a third direction of the seismic energy received at aseismic receiver as a function of time, the first, second and thirddirections not being co-planar; and the combined amplitude of theseismic data is generated from the amplitudes in the first, second andthird directions. This embodiment provides a combined three-componentamplitude of the seismic data.

Further preferred features of the present invention are set out in thedependent claims.

Preferred embodiments of the present invention will now be described byway of illustrative example with reference to the accompanying figuresin which:

FIG. 1 is a schematic illustration of the survey geometry for a VSPseismic survey;

FIG. 2 is a schematic illustration of the amplitude in one direction ofseismic energy received at the receiver of the VSP survey arrangementshown in FIG. 1;

FIG. 3(a) shows the variation with offset of the amplitude in thez-direction of seismic energy incident on a receiver in a typical VSPsurvey;

FIG. 3(b) shows the combined three-component amplitude corresponding tothe amplitudes in the z-direction shown in FIG. 3(a);

FIGS. 4(a) and 4(b) are partial enlarged views of FIGS. 3(a) and 3(b)respectively;

FIGS. 5(a) and 5(b) are partial further enlarged views of FIGS. 3(a) and3(b) respectively;

FIG. 6 shows the results of a time picking method of the presentinvention;

FIG. 7 is a flow chart illustrating one embodiment of a method of theinvention; and

FIG. 8 is a block diagram of a data processor suitable for carrying outthe present invention.

The effect of the method of the present invention is illustrated inFIGS. 3(b) and 4(b). These figures show a combined three-componentamplitude for the received seismic energy, calculated from theamplitudes in the x-direction, y-direction and z-direction. Thethree-component amplitude traces shown in FIG. 3(b) correspond to thesingle component traces shown in FIG. 3(a), and the three-componentamplitude traces of FIG. 4(b) correspond to the traces of FIG. 4(a). Ascan clearly be seen in FIGS. 3(b) and 4(b), the three-componentamplitude traces do not show any significant decrease in the amplitudeof the direct pulse as the offset changes. Determining the arrival timeof the direct pulse from the three-cormponent amplitude of the receivedseismic energy can therefore be carried out reliably by an algorithm orother automatic method.

In a preferred embodiment of the invention the combined three-componentamplitude of the seismic energy received at the receiver is calculatedas the sum of the three one-dimensional Hilbert instantaneous amplitudesof the received seismic energy. In this embodiment the time-dependentcombined three component amplitude A(t) is given by the followingequation: $\begin{matrix}{{A(t)} = {\sum\limits_{n = 1}^{3}{H_{A}\left( {d_{n}(t)} \right)}}} & (1)\end{matrix}$In this equation, t represents time, H_(A) is the one-dimensionalHilbert amplitude, and d_(n) with n=1, 2, 3 are the components of theseismic data in three orthogonal dimensions such as the x-, y- andz-directions.

The combined three-component amplitude A(t) is the total amplitudewaveform, and contains the amplitude of all the received seismic energyincident from every direction.

Once the three-component amplitude has been calculated, it is possibleto carry out the time picking step using a conventional time-pickingmethod. One conventional picking method is to calculate where thetangent to the amplitude of the received seisnic energy at the point ofinflection in the rise of the direct pulse crosses the zero amplitudeline. It is possible to apply this conventional picking method to thecombined three-component amplitude of the present invention. The resultsof this are indicated by the points 10 in FIG. 5(b), which is a furtherenlarged partial view of FIG. 3(b). The points 10 representing theresults of the conventional time pick are also indicated in FIG. 4(b).

Although it is possible to apply conventional time picking methods tothe three-component amplitude of the present invention, preferredembodiments of the present invention provides alternative time-pickingmethods.

According to one embodiment of the present invention, the time-pickingis carried out by determining the maximum positive gradient of thecombined three-component amplitude. That is, in this embodiment thearrival time of the direct pulse is defined to be the time at which:$\begin{matrix}{{\frac{\mathbb{d}{A(t)}}{\mathbb{d}t} > 0},{\frac{\mathbb{d}{A(t)}}{\mathbb{d}t} = \max}} & (2)\end{matrix}$

The arrival times derived by the maximum positive amplitude time pickingmethod are illustrated in FIG. 5(b) as the points 11, and is also shownin FIG. 4(b) by the line 11. This time pick is hereinafter referred toas the “maximum gradient pick”.

One possible disadvantage of identifying the arrival time of the directpulse to be the maximum positive gradient of the combinedthree-component amplitude is that it could possibly be susceptible tonoise in the seismic data. As shown schematically In FIG. 2 a seismictrace can contain noise, and any noise will make a contribution to thecombined three-component amplitude. If the noise in the combinedthree-component amplitude should have a greater positive gradient thanthe direct pulse, then the picking algorithm would wrongly identify thenoise as the direct pulse. In order to eliminate or reduce thepossibility of false time picks from this cause, in a particularlypreferred embodiment of the invention the conditions of equation (2)above are supplemented by a third condition that the three-componentamplitude is greater than a threshold value. That is, the arrival timeof the direct pulse is given by the time satisfying the followingequations: $\begin{matrix}{{\frac{\mathbb{d}{A(t)}}{\mathbb{d}t} > 0},{\frac{\mathbb{d}{A(t)}}{\mathbb{d}t} = \max},{{A(t)} \geq {A_{thresh}.}}} & (3)\end{matrix}$In one embodiment the threshold amplitude, A_(thresh) is defined to be aproportion of the maximum amplitude of the combined three-componentamplitude of the seismic data. That is:A _(thresh) =b×A _(max)  (4)In equation (3) b is a predetermined constant such that 0≦b≦1. Thisprovides a convenient way of defining the threshold amplitude.

It has been found that choosing b=0.25, so that A_(thresh)=0.25×A_(max),works well for most seismic data sets, although the threshold could beset higher if the data are particularly noisy.

An alternative embodiment of the present invention provides anothermethod of determining the arrival time of the direct pulse from thecombined three-component amplitude. In this embodiment the maximumpositive gradient of the three-component amplitude is determined, as inthe previous embodiment. Rather than identifying the arrival time of thedirect pulse to be the time at which the maximum positive gradientoccurs, however, in the alternative embodiment the amplitude of thethree-component amplitude is extrapolated from the time at which themaximum positive gradient occurs, back to zero amplitude. Theextrapolation is done using the determined value of the maximum positivegradient, and the arrival time of the direct pulse is identified to bethe time at which the extrapolated amplitude reaches zero. The resultsof this pick, hereinafter referred to as the “zero-crossing” pick, areshown by the lines 12 in FIGS. 4(b) and 5(b).

If the two picks of the present invention—that is the zero-crossing pickand the maximum gradient pick—are compared with the conventional pick,it will be seen that the maximum gradient pick occurs later in the tracethan either the zero-crossing pick of the present invention or theconventional point of inflection pick. It is believed that the timederived by the maximum gradient pick corresponds to the arrival time ofthe dominant frequency in the direct pulse. The maximum gradient pick ofthe present invention is a very well defined pick, and can be used as aseed pick for conventional picking.

At first sight, it appears that the zero-crossing pick 12 of the presentinvention is a good pick for determnining the time of the first receivedseismic energy at the receiver. However, tests have shown that, owing tointerference from reflections and mode conversions occuring near thereceiver, this pick is generally no better than the conventional pick.

The times determined by the three time picking methods are plotted onFIGS. 4(a) and 5(a), for comparison with the single component seismicdata. They are also shown on FIGS. 3(a), 3(b), 4(a) and 4(b).

A preferred embodiment of the present invention is describedschematically in the flow chart of FIG. 7.

At step 20 a seismic source in a VSP seismic survey is actuated to emita pulse of seismic energy. At step 21 the time-dependent amplitudetraces of the three components of the seismic energy received at aseismic receiver are recorded and stored. At step 22 the Hilbertinstantaneous amplitude trace of each of the input components of theseismic energy is calculated, and at step 23 the three Hilbertinstantaneous amplitudes are summed to determine the combinedthree-component amplitude A(t) using equation (1). This is also stored.

At step 24 the maximum value of the combined three-component amplitudeA(t) is determined. At step 25 a threshold value A_(thresh) isdetermined from the value of A_(max) in this example by multiplyingA_(max) by a predetermined constant b (i.e. using equation (4) above).

At step 26 a time is selected for which the instantaneous value of thethree-component amplitude trace A(t) is greater than the threshold valueA_(thresh). The value, at this selected time, of the second derivativewith respect to time of the combined three-component amplitude is thendetermined at step 27.

At step 28 it is tested whether the determined value of the secondderivative of A(t) at the selected time is equal to zero. If the resultof this determination is “yes”, this indicates that the firstderivative, with respect to time of A(t) is at a maximum, and the valueof the first derivative of A(t) at this time is calculated at step 32,and is stored. It is checked at step 33 that the value of dA(t)/dt ispositive; if the value of dA(t)/dt is found to be negative a new time isselected and steps 27 and 28 are repeated.

If it is found at step 28 that the second derivative of A(t) at theselected time is not equal to zero, at step 29 it is tested whether thesecond derivative of A(t) at the selected time is greater than zero. Ifthe result of this determination is “yes”, a new time, greater than theinitial chosen time is selected at step 31, and steps 26, 27 and 28 arerepeated. If the result of the determination in step 29 is “no”, a timeearlier than the initial chosen time is selected at step 31, and steps26, 27 and 28 are repeated. Steps 27 to 30 or 31 are repeated until a“yes” determination is achieved at step 28.

Once a “yes” determination is achieved at box 28, the value of the firstderivative of the three-component amplitude is calculated for the timeat which its second derivative is zero is calculated at step 32. At step33 it is checked whether the value if the first derivative is positive.If this step gives a “yes” determination, then the calculated value ofthe first derivative of the three component amplitude is known to be themaximum positive value of the gradient of the three-component amplitude.This value is stored.

At step 34 the zero-crossing time is determined, by extrapolatingbackwards from the time at which the second derivative of the combinedamplitude is zero, at the determined value of the maximum gradient. Thezero crossing time is then stored.

The time picking step is then concluded at step 35. The results of thetime picking step may then be used in further processing of the seismicdata.

In the embodiment described in FIG. 7, the combined three-component ofthe seismic data is computed at step 23 for the whole of the trace. Inpractice, an operator may have some idea of the likely arrival time ofthe direct pulse and, if so, it is not necessary to compute thethree-component amplitude of the seismic data for the entire trace.Instead, it is sufficient to compute the three-component amplitude ofthe seismic data for a time range which includes the expected arrivaltime of the direct pulse, for example the first half of the trace. Thisreduces the amount of processing required, and so reduces the time takento process the seismic data. If the three-component amplitude iscomputed only for a particular time range at step 23, it is of courseonly necessary at step 22 to compute the Hilbert instantaneous amplitudeof each component for this time range.

In the method shown in FIG. 7, in step 26, the trace is scanned fromtime t=0 to find a point at which the total three-component amplitudeexceeds the threshold amplitude.

However, if a trace contains noise at low times, a modified procedurecan be adopted to reduce the possibility that this noise at low timeswill produce a spurious time pick.

The modified procedure makes use of the parameter t_(noise), which ischosen such that the trace contains only for times in the range0<t≦t_(noise). Rather than processing data from t=0 in steps 22 and 23,the data is processed only for times t>t_(noise), thereby reducing theamount of processing required.

In this embodiment it is possible to define an alternative thresholdvalue for A(t). This is done by calculating the maximum amplitude of thenoise signal, A_(max-noise), in the time range up to t_(noise). Thealternative threshold for A(t) is then defined by:A _(thresh) =A _(max-noise) +b×(A _(max) −A _(max-noise))  (5)In equation (5) b is again a constant selected by the operator.

FIG. 6 shows results of applying the time picking method of the presentinvention to a three-dimensional VSP survey involving 21,240source/receiver pairs. In each case, the offset between the source andthe receiver was equal to or less than the depth of the receiver.

In FIG. 6, each short line represents five source/receiver pairs. Twosurveys were carried out, both in the same well. The results of onesurvey are shown in black in FIG. 6, and the results of the other surveyare shown in grey. A 99.4% success rate is estimated for the timepicking step, based on a count of cases in which the travel timeresidual varies by less than 100 ms from a calibrated one-dimensionalmodel.

The methods of processing seismic data described above can be carriedout using any conventional seismic data processing system. Theprocessing is preferably performed on a data processor configured toprocess large amounts of data.

FIG. 8 illustrates a data processor suitable for performing the methodof the present invention. The system comprises a programmable dataprocessor 40 with a programmable memory 41, for instance in the form ofa read—only memory ROM, storing a programme for controlling the dataprocessor 40 to perform, for example, the method illustrated in FIG. 7.The system further comprises non-volatile read/write memory 42 forstoring, for example, any data which must be retained in the absence ofa power supply. A “working” or “scratch pad” memory for the dataprocessor is provided by a random access memory RAM 43. An inputinterface 44 is provided, for instance for receiving seismic data,either direct from a receiver data or via an intermediate storagemechanism such as magnetic tape or discs. An output interface 45 isprovided, for instance for displaying and/or outputting the results ofthe data processing.

While preferred embodiments of the present invention have been describedabove, it should be understood that the descriptions and drawings areonly illustrative of the invention and are not intended to limit thescope of the present invention.

For example, in the preferred embodiment the time picking is carried outon a combined three-component amplitude derived from three components ofthe seismic energy incident on the receiver. In principle, however, thetime picking could be carried out on a combined amplitude generated fromtwo components of the seismic data. This can be done, for example, byperforming the summation in equation (1) for n=1,2 only, rather than forn=1, 2 and 3 as in the embodiments described above.

The zero-crossing time picking method and the maximum gradient timepicking method described above are not, in principle, limited to use ona combined two-component or three-component amplitude trace. Inprinciple, these picking methods can be applied to conventionalsingle-component traces.

1. A method of processing seismic data comprising the steps of: a)recording the amplitudes in first, second, and third directions ofseismic energy received at a seismic receiver as a function of time, thefirst, second, and third directions not being co-planar; b) generating atime-dependent combined amplitude A(t) of the seismic data from theamplitude in the first, second, and third direction, using${{A(t)} = {\sum\limits_{n = 1}^{3}{H_{A}\left( {d_{n}(t)} \right)}}};$wherein t represents time, H_(A) is the one dimensional Hilbertamplitude, and d_(n) with n= 1, 2, 3 are the components of the seismicdata in three orthogonal dimensions such as the x-, y- and z-directions, and c) time-picking the seismic energy arrival time from thecombined amplitude.
 2. A method as claimed in claim 1 wherein step (c)comprises time-picking the arrival time at which dA(t)/dt has itsgreatest positive value.
 3. A method as claimed in claim 2 wherein step(c) comprises time-picking the arrival time at which A(t) satisfies thefollowing equations:${\frac{\mathbb{d}{A(t)}}{\mathbb{d}t} > 0},{\frac{\mathbb{d}{A(t)}}{\mathbb{d}t} = \max},{{A(t)} \geq {A_{thresh}.}}$4. A method as claimed in claim 3 wherein A_(thresh) is given by:A _(thresh) =b×A _(max) where A_(max) is the maximum value of A(t), andb is a constant such that 0≦b≦1.
 5. A method as claimed in claim 3wherein A_(thresh) is given by:A _(thresh) =A _(max−noise) +b×/(A _(max) =A _(max−noise)) where A_(max)is the maximum value of A(t), A_(max−noise) is the maximum value of A(t)in the time range 0≦1≦1_(noise), and b is a constant such that 0≦b≦1. 6.A method as claimed in claim 5 wherein t_(noise) is selected such thatsubstantially no noise occurs in the seismic data for t>t_(noise).
 7. Amethod as claimed in claim 4 wherein b=0.25.
 8. A method as claimed inclaim 2, further comprising the step of extrapolating from the value ofA(t) at the time at which dA(t)/dt has its greatest positive value tothe time at which A(t)=0, the extrapolation being carried out using thegreatest positive value of dA(t)/dt.
 9. A method of processing seismicdata comprising the steps of: a) recording the amplitudes in first,second, and third directions of seismic energy received at a seismicreceiver as a function of time, the first, second, and third directionsnot being co-planar; b) generating a time-dependent combined amplitudeA(t) of the seismic data from the amplitudes in the first, second, andthird directions using${{A(t)} = {\sum\limits_{n = 1}^{3}{H_{A}\left( {d_{n}(t)} \right)}}};$and c) time-picking the seismic energy arrival (time from the combinedamplitude, wherein step (b) comprises generating the combined amplitudeof the seismic data only for seismic data received within apredetermined range of time.
 10. A method as claimed in claim 9 whereinstep (b) comprises generating the combined amplitude of the seismic dataonly for seismic data received at times t<t_(c) where t_(c) is less thenthe total time for which seismic data is recorded at the receiver.
 11. Amethod as claimed in claim 1 wherein the first and second directions aremutually perpendicular.
 12. A method as claimed in claim 1 wherein thefirst, second and third directions are mutually perpendicular.
 13. Amethod as claimed in claim 1 wherein the seismic data is VSP seismicdata.
 14. A method as claimed in claim 1 wherein step (c) comprisestime-picking the arrival time of a pulse of seismic energy transmittedfrom a seismic source to the receiver without an intermediatereflection.
 15. A method as claimed in claim 2 wherein step (c) furthercomprises time-picking the arrival time of a pulse of seismic energytransmitted from a seismic source to the receiver without anintermediate reflection.
 16. A method as claimed in claim 9 wherein step(c) comprises time-picking the arrival time at which dA(t)/dt has itsgreatest positive value.
 17. A method as claimed in claim 16, furthercomprising the step of extrapolating from the value of A(t) at the timeat which dA(t)/dt has its greatest positive value to the time at whichA(t)=0, the extrapolation being carried out using the greatest positivevalue of dA(t)/dt.
 18. A method of processing seismic data comprisingthe steps of: a) recording the amplitudes in first, second, and thirddirections of seismic energy received at a seismic receiver as afunction of time, the first, second, and third directions not beingco-planar; b) generating a time-dependent combined amplitude A(t) of theseismic data from the amplitudes in the first, second, and thirddirections using${{A(t)} = {\sum\limits_{n = 1}^{3}{H_{A}\left( {d_{n}(t)} \right)}}};$and c) time-picking the seismic energy arrival time from the combinedamplitude, wherein step (b) comprises generating the combined amplitudeof the seismic data only for seismic data received within apredetermined range of time and step (c) comprises time-picking thearrival time of a pulse of seismic energy transmitted from a seismicsource to the receiver without an intermediate reflection.
 19. A methodas claimed in claim 18 wherein step (c) comprises time-picking thearrival time at which dA(t)/dt has its greatest positive value.
 20. Amethod as claimed in claim 19, further comprising the step ofextrapolating from the value of A(t) at the time at which dA(t)/dt hasits greatest positive value to the time at which A(t)=0, theextrapolation being carried out using the greatest positive value ofdA(t)/dt.